types of histone MODIFICATIONS
Histone components of chromatin is subjected to three post synthetic modification which have either directed or indirect effect on eukaryotic gene regulation:
1. Histone methylation: affects only H3 and H4 and involves irreversible methylation of a few lysine residues which alters the hydrophobic nature of the side chain of these histones.
2. Histone phoshporylation: Involves Histone H1 phosphorylation. Phosphorylation affects serine and threonines, changing them from state of neutral charge to one of the negative charges and is a reversible reaction. The state of phosphorylation of H1 protein varies through the eukaryotic cell cycle and after H1 phosphorylation, chromatin becomes much more strongly condensed, as it does in mitotic chromosomes.
3. Histone acetylation: It is both reversible and irreversible type of modification of hisotnes. The 1st is the irreversible acetylation of the amino terminal serines of histones H1, H2A and H4. These modifications are associated with histone synthesis. The 2nd is reversible acetylation of lysine residues in the amino terminal group regions of H2A, H2B, H3 and H4 can have up to four lysine in acetylation form, there is a strong evidance that there is a correlation between this type of histone acetlation, easpecially tetracetylation of all available lysines and transcriptionally active chromatin. Evidently, the acetylation of core histone lysine would tend to loosen the nucleosomal structure which brings about the transition form a silent condensed gene to transcriptionally active and extended.
All such modification might occur during stressed condition and leads to gene regulation. [Verma and Agarwal 2010 referance book]
1. Histone methylation: affects only H3 and H4 and involves irreversible methylation of a few lysine residues which alters the hydrophobic nature of the side chain of these histones.
2. Histone phoshporylation: Involves Histone H1 phosphorylation. Phosphorylation affects serine and threonines, changing them from state of neutral charge to one of the negative charges and is a reversible reaction. The state of phosphorylation of H1 protein varies through the eukaryotic cell cycle and after H1 phosphorylation, chromatin becomes much more strongly condensed, as it does in mitotic chromosomes.
3. Histone acetylation: It is both reversible and irreversible type of modification of hisotnes. The 1st is the irreversible acetylation of the amino terminal serines of histones H1, H2A and H4. These modifications are associated with histone synthesis. The 2nd is reversible acetylation of lysine residues in the amino terminal group regions of H2A, H2B, H3 and H4 can have up to four lysine in acetylation form, there is a strong evidance that there is a correlation between this type of histone acetlation, easpecially tetracetylation of all available lysines and transcriptionally active chromatin. Evidently, the acetylation of core histone lysine would tend to loosen the nucleosomal structure which brings about the transition form a silent condensed gene to transcriptionally active and extended.
All such modification might occur during stressed condition and leads to gene regulation. [Verma and Agarwal 2010 referance book]
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